If a mix design requires 1700 lb of coarse aggregate at SSD with 0.9% moisture absorption, what amount needs to be weighed?

To determine the correct amount of coarse aggregate to weigh given that it has a moisture absorption of 0.9%, it's essential to first understand what "SSD" (Saturated Surface Dry) means and how moisture absorption affects the weight of the aggregate.

When the mix design specifies 1700 lb of coarse aggregate at SSD, this weight is based on the aggregate having no excess moisture—meaning it's held in a state where the water content is balanced. However, since the aggregate can absorb moisture and has a moisture absorption rate of 0.9%, it's crucial to adjust the weight to reflect the dry weight needed for the mix.

The formula to calculate the adjusted weight of the coarse aggregate can be expressed as:

Weight to be weighed = Required SSD weight / (1 + absorption rate)

In this context, the absorption rate is expressed in decimal form, so 0.9% becomes 0.009.

Substituting in the values:

Weight to be weighed = 1700 lb / (1 + 0.009)

Weight to be weighed = 1700 lb / 1.009

Weight to be weighed ≈ 1685 lb

Thus, the adjusted amount needed to account for the moisture absorption is